Conventional airbag inflators are relatively complex structures with such as forged housings defining internal ignition, combustion, and filter chambers by integrally formed and/or welded internal partitions. Furthermore, coolant structures, such as filters formed from heat conductive materials and the like, in many cases require the foregoing structural complexities in order to withstand the temperatures and pressures generated within these inflator structures.
Many such conventional inflators use azide based gas generating materials such as sodium azide based materials which have relatively high burn rates and undesirable toxicity levels and products of combustion such as mists and ash associated therewith.
Accordingly, there is a need in the prior art for more simplistic inflator structures, such as those formed from sheet metal having internal chambers formed in part by improved coolant/filter structures and utilizing non-azide propellants having controllable burn rates, gas volume production, internal pressures, and internal temperatures to increase the effectiveness of airbag inflators while reducing the size and the cost thereof and producing lesser amounts of undesirable products of combustion such as mists and ash.
The azide-based gas generating material (NaN.sub.3 /CuO, for example) has a relatively high linear burning velocity of about 45-50 mm/sec under the pressure of 70 kg/cm.sup.2. Because of the relatively high linear burning velocity, the azide-based gas generating material, even in the form of relatively large pellets or disk-shaped pieces with an excellent shape retention capability, can satisfy the required complete combustion time of 40-60 msec when used, for example, in the airbag inflator for the airbag at the driver's seat.
Non-azide gas generating materials have been developed which are excellent in terms of impacts on environment and safety of passengers. Such materials, however, have the linear burning velocity of less than 30 mm/sec in general. If we assume that the linear burning velocity is about 20 mm/sec and that the gas generating material is manufactured in the form of pellets 2 mm in diameter or disks 2 mm thick, which are advantageous in retaining their shapes, the combustion speed will be about 100 mm/sec, which fails to meet the desired combustion time of 40-60 msec. When the linear burning velocity is approximately 20 mm/sec, to obtain the desired combustion time requires the material's pellet diameter or disk thickness to be about 1 mm. When the linear burning velocity is less than 10 mm/sec, the gas generating material's disk is required to have a thickness of 0.5 mm or less. Thus, it is practically impossible to manufacture the gas generating material in the form of pellets or disks that are industrially stable and can withstand many hours of vibrations of an automobile. It has been difficult to develop the airbag inflator that meets the desired performances.
By way of specific example, reference is made to FIG. 9 wherein is shown a conventional airbag inflator such as disclosed in U.S. Pat. No. 4,547,342 of Adams et al., Oct. 15, 1985.
A housing 40 has a diffuser shell 41 and a closure shell 42. The diffuser shell 41 is formed by forging and has three concentric cylinders 43, 44, 45 formed integral with a circular portion 46. Like the diffuser shell 41, the closure shell 42 is also formed by forging and has three concentric welded portions 50, 51, 52. The diffuser shell 41 and the closure shell 42 are joined together at these welded portions 50, 51, 52 by friction welding. It is common with the prior art to form the shells of the airbag inflator by forging.
In this airbag inflator, the cylinder 43 defines an ignition means accommodating chamber 53, the cylinder 44 defines a combustion chamber 54, and the cylinder 45 defines a coolant/filter chamber 55. The ignition means accommodating chamber 53 accommodates an ignition means comprising an igniter 56 and a transfer charge 47. In the combustion chamber 54 there are installed pellets of a gas generating material 57, ignited by the ignition means to produce a gas, and a first coolant/filter 58 surrounding the gas generating material 57 to cool the combustion gas and arrest combustion particulates. In the coolant/filter chamber 55 is installed a second coolant/filter 59 to further cool the combustion gas and arrest combustion particulates.